An important challenge to the development of the advanced switching mode power supply is to increase the switching frequency of the power switches, because high switching frequency may decrease the size and the weight of a switching mode power supply. However, higher switching frequency results in higher switching loss, and it is therefore required to reduce the switching loss for implementing high switching frequency designs. FIG. 1 shows a conventional quasi-resonance flyback power supply 100, in which a power switch SW1 is serially connected to a primary coil P1 of a transformer TX between a power input Vin and ground GND, a capacitor C1 is shunt to the primary coil P1, and a control circuit 102 switches the power switch SW1 to produce a current on a secondary coil S1 of the transformer TX, in order to charge a capacitor C2 to produce an output voltage Vout.
FIG. 2 is a waveform diagram of the voltage across the power switch SW1 of the power supply 100 shown in FIG. 1. Between time t1 and t2, the power switch SW1 is off, a current flows from the secondary coil S1 through a diode D1 to charge the capacitor C2, and the voltage across the power switch SW1 will clamp to a predetermined value. Between time t2 and t3, the power switch SW1 is still off, the current on the secondary coil S1 is off, and the voltage across the power switch SW1 is resonated due to the presence of the capacitor C1, and thereby produces sinusoidal wave. Between time t3 and t4, the power switch SW1 is on, the voltage across the power switch SW1 falls down. To reduce the switching loss of the power switch SW1, the best timing to turn on the power switch SW1 is when the voltage across the power switch SW1 is at a minimum, that is, the valley of the sinusoidal wave.
Therefore, the key factor of reducing the switching loss is to precisely detect the minimum of the voltage across the power switch SW1. Usually, differentiators are used to detect the minimum of the voltage across the power switch SW1, for example, disclosed by U.S. Pat. No. 6,722,989 to Majid et al. According to the present invention, it is desired to detect the minimum of the voltage across the power switch SW1 without using any differentiator.